1. Field of the Invention
The present invention relates to a method of integrating the cement production process with mass burn technology by way of merging the hot gas evolved from the cement kiln into a mass burn system typified by incineration of municipal solid waste and industrial wastes in a rotary kiln.
2. Description of the Related Art
Even as society progresses economically and socially there is still a price for everything and among these are the huge prices incurred for waste disposal. Along with the higher standards of living the amount of waste requiring disposal have been increasing at an alarming rate especially in developed countries. Although measures have been in place including source reduction and recycling to reduce waste generated by a community, a major portion of the waste still needs to be disposed of in landfills and incineration. Waste incineration plants have been widely accepted and waste burning in cement kilns is also gaining increasing favour worldwide as an important waste management option. Advanced pollution control technology has also ensured that emission levels are maintained well below regulatory limits for public acceptance of these plants.
In a conventional (or dedicated) incineration plant the waste is burnt at a relatively low temperature and emissions originating from the waste are comparatively high in toxic levels. In a cement kiln burning waste as an alternative fuel, waste is introduced directly into the cement kiln where it is utilised as a fuel substitute and an additional source of energy. The present invention involves integration of the existing cement production process with the mass burn process in a new co-combustion process.
The cement manufacturing process is a series of energy intensive unit operations which are characterised by high temperature, long residence time, natural alkaline environment, minimum amount of waste generated and high thermal capacity. The main raw material used in the manufacture of cement is a well defined mix of Limestone, Copper Slag, Silica and Fly Ash. The raw mix as it is called is crushed in a grinding mill to achieve the required fine powder which is called the raw meal which is then homogenised and fed from the top of the preheater system—a string of cyclones arranged in stages. In this four stage preheater system the raw meal is heated up step-wise in a rising counter current stream of hot gas. The hot gas is generated by the burning of coal fuel at the firing end of the rotary kiln. In an intermediate process the raw meal is heated up to 900° C. to the calcination point of Limestone in a reactor named as Precalciner. After calcination is completed the raw meal is discharged into the feed end of the rotary kiln and heated up further. In the rotary kiln chemical and mineralogical changes take place as the material passes through further heating zones of calcining, transition, sintering and cooling. Sintering zone can reach up to temperatures of 1500° C. before the material is discharged as a semi-product called clinker. The clinker is then cooled in a grate cooler followed by cooling in a G-cooler. The cooled clinker is then mixed with gypsum and after a final grinding and separation process the cement product is obtained.
The mass burning process consists of incinerating unprocessed solid waste and thereby releasing its heat energy. The waste is burnt in a furnace wherein the bulk volume of waste is reduced. After the combustion process is completed the residue is discharged into a slag handling system. The hot gas from the combustion process is passed through a waste heat boiler where the gas is cooled down and steam is generated. Energy is recovered by passing the steam through a turbine and generation of electricity. The cooled gas is then passed into a flue gas treatment system in which pollutants are removed.
The inventor has recognised, according to the present invention, that the production of cement involves intensive combustion at high temperature and that most of the hot gas streams could be utilised for heat recovery and that the hot gas from the cement clinker coolers could be diverted and merged into a waste incinerator. The inventor has concluded that a co-combustion process can ensure destruction of dioxins to lower than the existing air emission regulation limit of 0.033 ng/Nm3 by optimum utilisation of the 3-Ts for combustion namely Temperature, Turbulence and Time:                Temperature—Incinerators are used extensively for mass burning because of the extremely high combustion efficiency and burnout in the kiln and have demonstrated high destruction and removal efficiency. At temperatures above 950° C. and up to 1200° C., complete destruction of the organic constituents of the fuel is obtained.        Turbulence—Turbulent mixing, which does not occur in a moving grate incinerator, gives the rotary incinerator a very high thermal efficiency in the recovery of heat from the waste feed material.        Time—Due to the length of the incinerator and the configuration of the secondary combustion chamber, longer retention time is ensured for reaction and complete burn out.        